1. Field of the Invention
The invention relates to a process for the synthesis of zeolites belonging to the faujasite structural family.
2. Background Art
Zeolites are crystalline tectosilicates. The structures consist of assemblages of TO.sub.4 tetrahedra forming a three-dimensional framework by the sharing of oxygen atoms. In zeolites of aluminosilicate type, which are the most common, T represents the tetravalent silicon and the trivalent aluminium. The abovementioned three-dimensional framework has cavities and channels which have molecular sizes and receive the cations which compensate for the charge deficiency related to the presence of trivalent aluminium in the TO.sub.4 tetrahedra, the cations generally being exchangeable.
Generally, the composition of the zeolites can be represented by the empirical formula (M.sub.2 /n O; Y.sub.2 O.sub.3 ; xZO.sub.2) in the dehydrated and calcined form. In this formula, Z and Y respectively denote the tetravalent and trivalent elements of the TO.sub.4 tetrahedra, M represents an electropositive element with a valency n, such as an alkali metal or alkaline-earth metal and constitutes the exchangeable balancing cation, and x is a number which can vary from 2 to theoretically infinity, in which case the zeolite is a silica.
Each type of zeolite has a distinct microporous structure. The variation in the sizes and shapes of the micropores from one type to another leads to changes in the absorbing properties. Only molecules having certain sizes and shapes are capable of entering into the pores of a specific zeolite. Due to their notable characteristics, zeolites are very particularly suited to the purification or the separation of gaseous or liquid mixtures such as, for example, the separation of hydrocarbons by selective adsorption.
The chemical composition, in particular the nature of the elements present in the TO.sub.4 tetrahedra and the nature of the exchangeable balancing cations, is also a significant factor which is involved in the adsorption selectivity and especially in the catalytic properties of these products. They are used as catalysts or catalyst supports in cracking, reforming and generally in modifying hydrocarbons, as well as in the synthesis of many molecules.
Many zeolites exist in nature: these are aluminosilicates, the availability and properties of which do not always correspond to the requirements of industrial applications. For this reason, the search for products having novel properties has led to the synthesis of a great variety of zeolites essentially of aluminosilicate type. Among the many examples of this type, it is possible to point out zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite L (FR-A-1,224,154), zeolite T (FR-A-1,223,775), zeolite ZSM (U.S. Pat. No. 3,702,886), zeolite ZSM12 (U.S. Pat. No. 3,832,449) and zeolite ZSM4 (EP-A- 0,015,132).
Zeolites of the faujasite structural family are characterized by a three-dimensional framework structure which can be described from the assemblage of units called cube-octahedra. Each of these units consists of 24 tetrahedra containing the elements Si and Al and bridged by oxygen according to the principle described above. In the cube-octahedron, the tetrahedra are joined so as to form eight rings containing six tetrahedra and six rings containing four tetrahedra. Each cube-octahedron is connected in tetrahedral coordination across four rings containing six tetrahedra to four neighbouring cube-octahedra.
It is convenient, in order to show the relationships which unite the various members of the structural family, to consider the structural planes in which the cube-octahedra are arranged at the vertices of a plane network of hexagons. Each cube-octahedron is thus connected to three neighhours in the structural plane.
The fourth bonding direction is directed alternately on either side of the structural plane and makes it possible to connect the cube-octahedra between neighbouring and parallel structural planes.
Depending on the relative arrangements of these structural planes with respect to one another, it is possible to obtain:
sequences of three distinct structural planes ABCABC . . . corresponding to a structure of cubic symmetry, PA1 sequences of two distinct structural planes ABAB . . . corresponding to a structure of hexagonal symmetry, PA1 more complex sequences which can be regular or irregular. PA1 for SiO.sub.2 /Al.sub.2 O.sub.3 =7, Na.sub.2 O/Al.sub.2 O.sub.3 will be taken between 1.4 and 2.4 PA1 for SiO.sub.2 /Al.sub.2 O.sub.3 =10, Na.sub.2 O/Al.sub.2 O.sub.3 will be taken between 1.8 and 2.8 PA1 for SiO.sub.2 /Al.sub.2 O.sub.3 =15, Na.sub.2 O/Al.sub.2 O.sub.3 will be taken between 2.2 and 3.5
All the solids belonging to the faujasite structural family are polytypes and have interconnected channels with a diameter of approximately 0.8 nm. Thus faujasite is a natural zeolite whose structure corresponds to the stacking of three distinct structural planes ABC corresponding to a structure of cubic symmetry. It is possible to obtain, by synthesis from a sodium aluminosilicate gel, compounds of the same structure as faujasite, the said compounds being called zeolites X when the Si/Al ratio of the number of silicon atoms to the number of aluminium atoms is between 1 and 1.5 and zeolites Y when the said Si/Al ratio is greater than 1.5.
The general process for the synthesis of zeolites of the faujasite structural family consists of a hydrothermal crystallization of aluminosilicate gels of specific compositions containing a structuring agent, which can be a metal cation and optionally a cation such as tetraethylammonium.
This synthesis has its limits. It does not make it possible to obtain either faujasites having Si/Al ratios greater than 3 or virtually pure hexagonal faujasites. In fact, the faujasites obtained by this process have a cubic structure or else exist in the form of mixtures of cubic and hexagonal shapes.
French Patent 2,638,444 from Elf Aquitaine describes a process which makes it possible to obtain by direct synthesis faujasites whose Si/Al ratio is greater than 3.
These more acidic faujasites are obtained by a process using carbonaceous macrocycles containing heteroatoms chosen from oxygen, nitrogen or sulphur.
The use of these macrocycles makes it possible to direct the synthesis towards the production of cubic or hexagonal faujasites or a mixture of the two structures.
In fact, by using as structuring agent macrocycles having 10 to 17 atoms per ring, the cubic faujasite is obtained, whereas structuring agents containing 18 to 24 atoms in their ring direct the synthesis towards the hexagonal polytype.
In this process, the macrocycles are used in the purified form. Their purification requires many distillation and chromatographic separation stages.